1. Field of the Invention
The present invention relates to an apparatus and a method for providing a heat sink on a semiconductor chip. More particularly, the present invention relates to forming a heat sink on an upper surface of a semiconductor chip by placing a heat conductive material thereon which forms a portion of a glob top.
2. State of the Art
Chip On Board (xe2x80x9cCOBxe2x80x9d) techniques are used to attach semiconductor dice to a printed circuit board including flip chip attachment, wirebonding and tape automated bonding (xe2x80x9cTABxe2x80x9d).
Flip chip attachment consists of attaching a flip chip to a printed circuit board or other substrate. A flip chip is a semiconductor chip that has a pattern or array of terminations spaced around an active surface of the flip chip for face-down mounting of the flip chip to a substrate. Generally the flip chip active surface has one of the following electrical connectors: Ball Grid Array (xe2x80x9cBGAxe2x80x9d)xe2x80x94wherein an array of minute solder balls is disposed on the surface of a flip chip which attaches to the substrate (xe2x80x9cthe attachment surfacexe2x80x9d); Slightly Larger than Integrated Circuit Carrier (xe2x80x9cSLICCxe2x80x9d)xe2x80x94which is similar to a BGA but has a smaller solder ball pitch and diameter than a BGA; or a Pin Grid Array (xe2x80x9cPGAxe2x80x9d)xe2x80x94wherein an array of small pins extends substantially perpendicularly from the attachment surface of a flip chip wherein the pins conform to a specific arrangement on a printed circuit board or other substrate for attachment thereto. With the BGA or SLICC, the solder or other conductive ball arrangement on the flip chip must be a mirror image of the connecting bond pads on the printed circuit board such that precise connection is made. The flip chip is bonded to the printed circuit board by reflowing the solder balls. The solder balls may also be replaced with a conductive polymer. With the PGA, the pin arrangement of the flip chip must be a mirror image of the pin recesses on the printed circuit board. After insertion, the flip chip is generally bonded by soldering the pins into place. An underfill encapsulant is generally disposed between the flip chip and the printed circuit board for environmental protection and to enhance the attachment of the flip chip to the printed circuit board. A variation of the pin-in-recess PGA is a J-lead PGA wherein the loops of the J""s are soldered to pads on the surface of the circuit board.
Wirebonding and TAB attachment generally begin with attaching a semiconductor chip to the surface of a printed circuit board with an appropriate adhesive such as an epoxy. In wirebonding, a plurality of bond wires is attached one at a time to each bond pad on the semiconductor chip and extends to a corresponding lead or trace end on the printed circuit board. The bond wires are generally attached through one of three industry-standard wirebonding techniques: ultrasonic bondingxe2x80x94using a combination of pressure and ultrasonic vibration bursts to form a metallurgical cold weld; thermocompression bondingxe2x80x94using a combination of pressure and elevated temperature to form a weld; and thermosonic bondingxe2x80x94using a combination of pressure, elevated temperature and ultrasonic vibration bursts. The semiconductor chip may be oriented either face up or face down (with its active surface and bond pads either up or down with respect to the circuit board) for wire bonding, although face up orientation is more common. With TAB, ends of metal leads carried on an insulating tape such as a polyamide are respectively attached to the bond pads on the semiconductor chip and to the lead or trace ends on the printed circuit board. An encapsulant is generally used to cover the bond wires and metal tape leads to prevent contamination.
After assembly as shown in FIG. 1, a glob of encapsulant material 102 (usually epoxy or silicone or a combination thereof) is generally applied to a COB assembly 100 to surround a semiconductor chip or flip chip 104 which is attached to a substrate 106 via a plurality of electrical connections 108 which extends between a plurality of semiconductor chip bond pads 110 and a corresponding plurality of substrate bond pads 112. An underfill encapsulant 114 is dispensed between the semiconductor chip 104 and the substrate 106. As shown in FIG. 2, the glob top materials 202 are often used to hermetically seal bare dice 204 (shown in shadow) on a printed circuit board 206 such as SIMM modules to form a COB assembly 200. The organic resins generally used in the glob top encapsulation are usually selected for low moisture permeability and low thermal coefficient of expansion to avoid exposure of the encapsulated chip to moisture or mechanical stress respectively. However, even though the chemical properties of these glob top materials have desirable properties for encapsulation, the thermal and electrical properties are often not optimal for removing heat efficiently away from the semiconductor dice or for use in high temperature areas.
Every semiconductor chip in a COB assembly generates some heat during operation. Some glob tops and package encapsulation materials serve to draw the heat away from most semiconductor chips. Indeed, one factor in choosing a package encapsulation material is its thermal dissipation properties. If the temperature of the semiconductor chip is not controlled or accommodated, system reliability problems may occur due to excess temperature rise during operation. The device/semiconductor junction temperature (the location of the heat source due to power dissipation) must be maintained below a limiting value such as 85xc2x0 C. The primary reason to control this temperature is that switching voltage is a sensitive function of device temperature. In addition, various failure mechanisms are thermally activated and failure rates become excessive above the desired temperature limit. Furthermore, it is important to control the variation in device operating temperature across all the devices in the system. This is also due to the temperature sensitivity of switching voltage since too large a variation from device to device would increase the voltage range over which switching occurs, leading to switching errors due to noise and power-supply fluctuations. Moreover, the fluctuations in temperature cause differential thermal expansions which give rise to a fatigue process that can lead to cracks occurring in the COB assembly during burn-in or general operation.
Thus high heat-producing semiconductor dice such as microprocessors may require adjustments in the size of the COB assembly and will often require the addition of metal heat-dissipating fins, blocks or the like on the package. FIG. 3 illustrates a finned COB assembly 300. The finned COB assembly 300 comprises a semiconductor chip or flip chip 302 which is attached to a substrate 304 via a plurality of electrical connections 306 which extends between a plurality of semiconductor chip bond pads 308 and a corresponding plurality of substrate bond pads 310. An underfill encapsulant 312 is dispensed between the semiconductor chip 302 and the substrate 304. A cap 314 having a plurality of heat-dissipating fins 316 is attached to an upper surface 318 of the semiconductor chip 302 with a layer of thermally conductive adhesive 320. The addition of heat-dissipating fins, blocks or the like substantially increases the cost of production for COB assemblies.
Other means for heat dissipation have also been attempted. U.S. Pat. No. 5,434,105 issued Jul. 18, 1995, to Liou relates to the use of heat spreaders attached to a semiconductor device by a glob top to strengthen the heat coupling from an integrated circuit die to the lead frame wherein heat can then pass through the leads of the lead frame to the circuit board. However, the heat is not dissipated away from the circuit. Rather, the heat is conducted into the circuit board, which can still cause heat related problems. U.S. Pat. No. 5,488,254 issued Jan. 30, 1996, to Nishimura et al. and U.S. Pat. No. 5,489,801 issued Feb. 6, 1996, to Blish relate to encasing a heat slug (a piece of heat conducting material) in the encapsulation material. Although each of these patents attempts to address the problems of potential differences in the thermal coefficient of expansion between the heat slug and the encapsulation material, these attempts are never entirely successful and the adhesion interfaces between the heat slug and the encapsulation material may become separated, allowing moisture to reach and destroy the encased chip.
Changes in encapsulation materials have also been attempted to achieve high thermal conductivity, low coefficient of thermal expansion and low moisture permeability. U.S. Pat. No. 4,358,552 issued Nov. 9, 1982, to Shinohara et al. and U.S. Pat. No. 4,931,852 issued Jun. 5, 1990, to Brown et al. are examples of such attempts. However, no attempt has been entirely successful in balancing all of these desired factors or are simply too expensive.
U.S. Pat. No. 5,379,186 issued Jan. 3, 1995, to Gold et al. (xe2x80x9cGoldxe2x80x9d) relates to a heat-producing semiconductor chip attached to a substrate which uses multiple encapsulants to dissipate heat. xe2x80x9cGoldxe2x80x9d teaches placing a layer of encapsulant material over the semiconductor chip with a layer of thermally conductive material applied over the encapsulant material layer. xe2x80x9cGoldxe2x80x9d specifically teaches that the encapsulant material layer used for covering the semiconductor is a relatively poor conductor of heat (i.e., an insulative material) which is assumedly chosen for its adherence and protective properties. The thermally conductive material is applied over the encapsulant material to aid in the removal of heat from the semiconductor device through the insulating encapsulant material. However, this invention is inherently inefficient since the heat must be drawn from an insulative material.
Therefore, it would be advantageous to develop a technique and assembly for inexpensively forming a heat-dissipating mechanism on a semiconductor chip in combination with commercially available, widely practiced semiconductor device fabrication techniques.
The present invention relates to an apparatus and a method for providing a heat sink on a semiconductor chip. The apparatus is constructed with a two-step process for forming a dual material glob top. The process comprises providing a semiconductor chip attached to and in electrical communication with a substrate by any known industry technique such as flip chip attachment, TAB attachment, wirebonding and the like. A barrier glob top material is applied to the edges of the semiconductor chip on the surface (xe2x80x9copposing surfacexe2x80x9d) opposite the surface (xe2x80x9cattachment surfacexe2x80x9d) attached to the substrate to form a wall around a periphery of the opposing surface of the semiconductor chip and extends to contact and adhere to the substrate. The barrier glob top performs the function of sealing and protecting the semiconductor chip. Thus the barrier glob top material is selected for low moisture permeability, low thermal coefficient of expansion, and good adhesion and sealing properties. Preferred barrier glob top materials include epoxy, polyamide, urethane silicone, acrylic or the like.
If the semiconductor chip makes electrical contact between the opposing side and the substrate with bond wires or TAB, the wall formed around the periphery of the opposing surface preferably covers and encapsulates the bond wires or TAB. If the semiconductor chip is a flip chip, an underfill encapsulant may be disposed between the semiconductor chip and the substrate.
The wall around the periphery of the opposing surface of the semiconductor chip forms a recess. A heat-dissipating glob top material is disposed within the recess to contact the opposing surface of the semiconductor chip. The heat-dissipating glob top material is chosen for its ability to transfer heat away from the semiconductor chip (i.e., high thermal conductivity material). As a general matter, the heat-dissipating glob top material has a higher thermal conductivity than the barrier glob top material. The heat-dissipating glob top may also extend over the barrier glob top wall to contact the substrate. It is also understood that a plurality of semiconductor chips with barrier glob tops could be attached to a substrate with a continuous heat-dissipating glob top filling each semiconductor chip barrier glob top recess and covering each of the plurality of semiconductor chips. Preferred heat-dissipating glob top materials include: standard, high purity barrier glob top materials containing arsenic, boron, gallium, germanium, phosphorus, silicon or other such suitable highly conductive materials.
Differences in the thermal coefficient of expansion between the barrier glob top and the heat-dissipating glob top and the potential of separation of the interface between the barrier glob top and the heat-dissipating glob top are less an issue with the present invention since the barrier glob top completely seals the semiconductor chip from moisture or external contamination.
Thus, the apparatus of the present invention has all of the adherence and sealing benefits of a low thermal conductivity glob top material while at the same time enjoying the benefits of heat-dissipation provided by a high thermal conductivity glob top material.